1. Field of the Invention
The present invention relates to a nonaqueous electrolyte secondary cell using a nonaqueous electrolyte and positive and negative electrode active materials that are capable of reversibly intercalating lithium ions, and more particularly, to a nonaqueous electrolyte secondary cell having improved safety.
2. Description of the Prior Art
In recent years, rapid advancements in size reduction and weight reduction of mobile information terminals, such as mobile telephones and notebook computers, have created an increasing demand for nonaqueous electrolyte secondary cells, which are lightweight and have high capacity.
Nonaqueous electrolyte secondary cells perform charge and discharge by migration of lithium between the positive electrode and the negative electrode. Generally, nonaqueous electrolyte secondary cells use a carbon-based material that is capable of reversibly intercalating lithium ions (for the negative electrode active material), a transition metal oxide such as lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, and the like (for the positive electrode active material), and a nonaqueous electrolyte containing a lithium salt. Such nonaqueous electrolyte secondary cells exhibit excellent charge-discharge characteristics insofar as charge and discharge are performed in an appropriate range.
However, when the cells are overcharged, the lithium ions that cannot be stored in the negative electrode deposit on the negative electrode in the form of lithium metal, and the deposit develops into dendrites. The developed dendrites pierce through the separator and reach the positive electrode, causing an internal short circuit. In conventional nonaqueous electrolyte secondary cells, the dendrites fully grow and quickly pierce through the separator. This causes great damage to the separator and the resulting internal short circuit cause the cell temperature to rise to such a degree that cell performance is degraded.
Moreover, overcharge causes the positive electrode potential to increase (for example, to exceed more than 5 V), and as a result, decomposition of the electrolyte solution occurs on the positive electrode. Decomposition of the electrolyte solution induces a shortage of electrolyte solution and an increase of cell internal pressure, and when the cell temperature increase mentioned above occurs in addition to these, the electrode active materials and the electrolyte solution react violently.
In view of this problem, conventional nonaqueous electrolyte secondary cells incorporate, in order to ensure safety of the cells, separately-produced protective circuits such that, for example, electric current is cut off when cell voltage excessively increases. Such incorporation of protective circuits, however, increases the cost of the cells and moreover impedes reductions in size and weight of the cells.
In view of the foregoing and other problems in the prior art, it is an object of the present invention to improve safety of a cell, without incorporating a separately-produced protective circuit therein, by effectively utilizing an internal short circuit induced by dendrites of lithium metal, which is a mechanism inherent to the cell.
It is another object of the invention to improve safety of a cell while achieving reductions in size, weight, and cost of the cell, by self-containedly suppressing an increase in cell temperature and gas formation that is caused by overcharge without using special components.
These and other objects are accomplished in accordance with the present invention by providing a nonaqueous electrolyte secondary cell comprising a positive electrode, a negative electrode, a nonaqueous electrolyte, a separator interposed between the positive electrode and the negative electrode, the positive electrode having a positive electrode active material comprising a chemical compound capable of reversibly intercalating lithium and the negative electrode having a negative electrode active material comprising a material capable of reversibly intercalating lithium, wherein the separator has through holes for passing lithium dendrites therethrough.
In cases where a cell is overcharged and thereby lithium is released from the positive electrode in an amount exceeding the capacity of the negative electrode or where charge is performed in a low temperature condition in which the reactivity of the negative electrode is decreased, lithium dendrites deposit on the negative electrode. When the lithium dendrites pass through the separator and connect the positive electrode and the negative electrode to allow electrical contact therebetween at an initial stage of the lithium dendrite formation (before the dendrites fully grow), a short circuit is caused and thereby charge reaction does not further proceed. In addition, since the lithium dendrites are small in diameter at this stage, safety problems caused by, for example, increases in cell voltage and cell temperature, are suppressed.
Thus, as described above, when the separator has through holes for passing lithium dendrites therethrough, electrical contact between the positive electrode and the negative electrode is established at the initial stage of lithium dendrite formation, and consequently, safety of the cell is maintained.
In the above-described nonaqueous electrolyte secondary cell, the through holes may have a substantially straight line-shape and the positive electrode and the negative electrode may be connected thereby.
When the through holes have a substantially straight line-shape and the positive electrode and the negative electrode are connected thereby, lithium dendrites can smoothly grow and thereby electrical contact between the positive electrode and the negative electrode is formed at an earlier stage of lithium dendrite formation. Thus, safety of the cell is further improved.
In the above-described nonaqueous electrolyte secondary cell, the through holes may be such that the positive electrode and the negative electrode are connected in the shortest possible distance.
This configuration makes it possible to form electrical contact between the positive electrode and the negative electrode at an even earlier stage of lithium dendrite formation, and accordingly, safety of the cell is even further improved.
In the above-described nonaqueous electrolyte secondary cell, the through holes may have a diameter of 5 xcexcm or greater
When the diameter of the through holes is 5 xcexcm or greater, the positive electrode and the negative electrode are easily connected even when the lithium dendrites grow significantly in a transverse direction (in a direction parallel to the substrate).
In the above-described nonaqueous electrolyte secondary cell, the through holes may have a diameter of 100 xcexcm or less, and preferably 70 xcexcm or less.
When the diameter of the through holes is 100 xcexcm or less, the possibility of occurrences of internal short circuits is reduced under normal conditions of use (not in overcharge conditions or the like). When the diameter of the through holes is 70 xcexcm or less, it is ensured that internal short circuits are prevented under normal conditions of use.
In the above-described nonaqueous electrolyte secondary cell, the through holes may have a diameter of 50 xcexcm or less.
When the diameter of the through holes is 50 xcexcm or less, a shutdown mechanism smoothly works in which the separator, which is composed of a microporous film made of polyethylene or polypropylene, melts in cases where a cell temperature increase occurs and thereby prevents current flow between the positive electrode and the negative electrode. As a consequence, safety of the cell is further improved.
In the above-described nonaqueous electrolyte secondary cell, the through holes may have a diameter of 30 xcexcm or less.
When the through holes have a diameter of 30 xcexcm or less, cell degradation due to self-discharge can be suppressed, and accordingly, when the cell is stored at high temperature, cell voltage variation and cell thickness increase are small.
In the above-described nonaqueous electrolyte secondary cell, the through holes may be provided at a density of 1 through hole per square centimeter or more.
When the through holes are present at a density of 1 through hole/cm2 or more, it is possible to provide the electrical contact that is caused by the lithium dendrites formed randomly on the negative electrode at an earlier stage of the dendrite formation. In addition, since the number of the positions where electrical contact is formed is greater, each of the positions where the electrical contact is formed has a smaller load. Therefore, safety of the cell is further improved.
The above-described nonaqueous electrolyte secondary cell may further have a conductive polymer provided between the separator and the positive and negative electrode active materials.
In secondary cells, as charge-discharge cycles are repeated, deposit is produced by detachment of the active materials from the electrode plates and decomposition of the electrolyte solution. The deposit blocks the through holes in the separator and prevents the electrical contact that is formed by the lithium dendrites. When, as in the above-described configuration, a conductive polymer is present between the separator and the active materials, the deposition of the detached materials and the like does not occur and therefore, even when charge-discharge cycles are repeated, the electrical contact can be maintained at the same level as that of the cell in the initial condition immediately after the fabrication.
The present invention also provides a nonaqueous electrolyte secondary cell comprising a positive electrode, a negative electrode, a nonaqueous electrolyte, a separator interposed between the positive electrode and the negative electrode, the positive electrode having a positive electrode active material comprising a chemical compound capable of reversibly intercalating lithium and the negative electrode having a negative electrode active material comprising a material capable of reversibly intercalating lithium, wherein the separator comprises through holes having a diameter of 5 xcexcm or greater.
In the above-described nonaqueous electrolyte secondary cell, the through holes may have a diameter of 100 xcexcm or less, and more preferably have a diameter of 70 xcexcm or less.
In the above-described nonaqueous electrolyte secondary cell, the through holes may have a diameter of 50 xcexcm or less.